The present invention relates to an add/drop multiplexer for wavelength division multiplexing. The invention is especially directed to use with fibre optic cables, in particular to use as a branching unit adapted for use in a fibre optic network. The invention further relates to such fibre optic networks, particularly in the context of submarine cable systems employing fibre optic cables.
Wavelength division multiplexing, termed WDM, (discussed in, for example, Hill, British Telecom Technology Journal 6(3):24-31) is a technique of considerable benefit in optimising transmission of signals through fibre optic networks. In wavelength division multiplexing, traffic signals to be sent out by a station are modulated on to a number of carrier signals at different predetermined carrier wavelengths. Each predetermined carrier wavelength is allocated according to the identities of the send station and of the intended receive station. Predetermined carrier wavelengths will be spaced sufficiently far apart in wavelength that they can be discriminated from each other by components of the fibre optic system, but in many networks will need to be grouped sufficiently closely that all carrier wavelengths can be amplified satisfactorily by the same amplifier in a repeater (or in unrepeatered systems, to be carried long distances without significant loss). The carrying capacity of a single fibre is enhanced by WDM--rather than carrying a single signal, the fibre is simultaneously carrying several signals, each of a different wavelength.
Most such transmission networks have a number of nodes at which one or more branches form away from a main trunk or ring. Typically, at these nodes one or more carrier wavelengths are dropped down one fibre of the branch and one or more carrier wavelengths (which may be the same as, or different from ,those dropped from the trunk or ring) are added to the trunk or ring from another fibre of the branch. The component which performs such a function is an Add/Drop Multiplexer (ADM).
WDM is particularly well adapted to efficient routing of signals between send and receive stations. As different signals have different carrier wavelengths, optical components can be used to route signals appropriately by directing them according to the carrier wavelength of the signal.
This can be done in an active manner, by splitting the signal into its component carrier wavelengths with a prism or similar component, and actively processing the routing the splint signals to desired outputs. This solution is appropriate for use in an integrated device: a basic design for a multiplexer of this type is discussed in Dragone et al in IEEE Photonics Technology Letters 3(10):896-899, and designs employing arrayed-waveguide gratings are disclosed for an ADM in Okamoto et al in Electronics Letters 31(9):723-4 and for an optical splitter/router in Inoue et al in Electronic Letters 31(9):726-7. A difficulty with such silicon-based components is a lack of flexibility: to perform a specific add-drop function for particular wavelengths, a specific device will need to be fabricated. In a network it will be necessary for different nodes to add, drop, or pass different combinations of carrier wavelengths: with integrated components of the type described, it may prove necessary to fabricate different components for each node. This could require a different mask to be prepared for each component, and would as a consequence be likely to be prohibitively expensive for a customized network.
Alternatively, essentially passive optical components can be used which respond differently to different carrier wavelengths. This enables an essentially passive network to be constructed.
An example of an appropriate wavelength-sensitive optical component is a fibre Bragg grating. Fibre Bragg gratings are discussed in Bennion et al, Electronics Letters, Vol. 22., 341-343, 1986. A Bragg grating is a notch reflection filter. Light is transmitted through the grating at all wavelengths apart from those falling within a narrow wavelength band. Light within the band is substantially totally reflected. With an appropriate spacing of carrier wavelengths in a WDM system, a fibre Bragg grating can be adapted to reflect only one of the carrier wavelengths and allow the others to pass.
An alternative approach to producing a practical ADM, employing fibre Bragg gratings, is proposed in Johnson et al in Electronics Letters 23(13):688-9. Refinements are shown in Cullen et al, Electronics Letters, Vol. 30, 2160-2162, 1994, and Bilodeau et al, IEEE Photonics Technology Letters, Vol. 7, 388-390, 1995. This ADM is an optical tap comprising a Mach-Zehnder interferometer. This optical tap is illustrated in FIG. 1. It comprises two input fibres 101, 102 and two output fibres 103, 104, two 3dB direction couplers 105 which split input light equally between output paths, and two interferometer arms of identical path length linking the coupler. In each arm there is a Bragg reflection filter 105 which passes light at wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, but which reflects light at .lambda..sub.0. Light at wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 passes from input 101 through both arms. It then combines constructively at output 103 and is transmitted out through it: however it combines destructively at input 102 (because of the phase shift introduced) and is not transmitted.